The invention relates to solid electrolyte capacitors of the type described in commonly assigned U.S. Pat. Nos. 5,136,478 (Bruder et al) issued Aug. 4, 1992 and 5,161,094 (Bruder et al), issued Nov. 3, 1992, and 5,047,899 issued Sep. 10, 1991, and more particularly to improvements in the method and structures for making and packaging such solid electrolyte capacitors.
The above-mentioned U.S. Pat. No. 5,136,478, entitled "Solid Electrolyte Capacitor and Method of Making", by Bruder et al, issued Aug. 4, 1992 is incorporated herein by reference. U.S. Pat. No. 5,136,478 discloses various solid-state electrochemical capacitors that include a layer of solid electrolyte material composed of RbAg.sub.4 I.sub.5, a layer of anode material composed of carbon and RbAg.sub.4 I.sub.5 adjoining a first surface of the electrolyte layer, and a layer of cathode material composed of carbon and RbAg.sub.4 I.sub.5 adjoining a second surface of the electrolyte layer. A charging current charges the anode to a voltage in the range from 0.50 to 0.66 volts to cause storage of charge in both a "double layer capacitance mode" and a "pseudocapacitance mode". The charging and discharging mechanisms as presently understood are described in detail with reference to FIGS. 16A and 16B of commonly assigned U.S. Pat. No. 5,136,478. The formation of silver "dendrites" during charging and discharging cycles of a solid electrolyte capacitor is described as being problematic. The most recently preferred structure is shown in FIG. 14 of U.S. Pat. No. 5,136,478 and the charging mechanism is described in the associated text. It has proven reliable for thick cells, but for thin cells silver that forms and is extruded where the silver anode contact layer 2A contacts the anode 2Layer 2E-1 is likely to cause an anode-to-cathode short.
As efforts toward providing a commercially useful structure capable of functioning as 15 to 30 volt energy sources have progressed, additional challenges have arisen. Often, fast charging times are required for such energy sources. The individual anode-electrolyte-cathode disks described in commonly assigned U.S. Pat. No. 5,136,478 must be stacked in a unitary packaging structure to provide full charge voltage levels of 1.3 to 30 volts. To obtain satisfactorily high energy density of such an energy source, ways must be found to make each solid electrolyte capacitor disk as thin as possible while maintaining high resistance of each disk to mechanical shock, avoiding reliability problems caused by the formation of dendrites and/or migration of silver as a result of repetitive charge/discharge cycles and the resulting electrical shorting of the cathode to the anode. For some applications, maintenance of very low self-discharge rates of such energy sources is necessary. It has been discovered that when a large number of individual "cells" of the type described in commonly assigned U.S. Pat. No. 5,136,478 are stacked in a single structure to provide a high full-charge voltage, uneven self-discharge in individual cells can result in overcharging some of the cells during subsequent charging cycles, which degrades cell performance. If overcharging beyond 0.66 volts occurs or if charging too rapidly between 0.625 and 0.66 volts occurs, iodide in the electrolyte layer becomes iodine, which migrates away from the cathode to carbon surfaces, leaving the electrolyte depleted of iodide, reducing the capacitance of the cell. It would be desirable for the case or housing in which a number of individual solid electrolyte capacitor cells are stacked to have the same general appearance and electrode polarities as a conventional dry cell.
Thus, the developments described in the above-mentioned commonly assigned patents have led to a need for a rugged packaging structure and method that provides a plurality of individual solid electrolyte capacitor cells in a single package, with conventional appearance and conventional voltage polarities associated with its terminals, provides rapid charge and discharge times and high power and energy densities, and avoids the various problems mentioned above.